<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>单列布局</title>
  <style>
    .header {
      margin: 0 auto;
      width: 960px;
      height: 100px;
      background-color: blueviolet
    }

    .content {
      margin: 0 auto;
      width: 960px;
      height: 500px;
      background-color: #ccc
    }

    .footer {
      margin: 0 auto;
      width: 960px;
      height: 100px;
      background-color: yellowgreen
    }
  </style>
</head>

<body>
  <header class="header"></header>
  <main class="content"></main>
  <footer class="footer"></footer>
</body>

</html>